The present invention relates to flexible films coated with pressure sensitive adhesive for releasable seating to a target surface, and more particularly to such films which have features for preventing premature sticking to a target surface during film positioning thereon. Even more particularly, the present invention relates to flexible films having protrusions formed on an adhesive side which act to space a pressure sensitive adhesive from a target surface until the film is pressed thereagainst. It further relates to a method of making such formed and adhesively coated flexible films.
In the art of tapes, labels, and other articles using pressure sensitive adhesive to adhere an adhesive coated surface to a target surface, there has been recognized the problem of premature sticking. That is, before the adhesive coated surface can be properly positioned over a target surface, inadvertent contact of the adhesive with the target surface causes premature sticking at one or more locations, thereby inhibiting proper positioning.
Others have attempted to overcome this problem. For example, in the catamenial field, where it is useful to adhere an absorbent pad to the inside of an undergarment, manufacturers have made it necessary to press the garment against the back of the pad before adhesive contact can occur. Protecting the adhesive from premature contact is achieved by adding strips of foam to the back of the pad, which provide an initial contact surface outwardly from the adhesive surface. The strips of foam act as standoffs from the adhesive surface. Once the undergarment is positioned properly, it may be pressed into the space between strips of foam. Adhesion to a pressure sensitive adhesive layer fixes the pad in place. Of course, for this approach to work, the undergarment has to be flexible enough to deform around the strips of foam.
U.S. Pat. No. 4,376,440 to Whitehead et al. discloses sanitary napkins having recessed patterns of pressure sensitive adhesives for sticking the napkins to undergarments. A water based emulsion adhesive is suggested. Foam baffles surround pockets into which the adhesive is placed. This invention is limited to target surfaces which deform into the adhesive pockets.
European Patent Application No. 0 623 332 A1 to Lauritzen describes methods for making a sanitary napkin which has a recessed adhesive pattern for attaching it to a user""s undergarment. Lauritzen forms depressions in a fluid-impervious barrier film. The depressions create raised areas in the barrier film on the side of the napkin which attaches to the undergarment. Adhesive is placed onto the barrier film side having the raised areas, but only in discrete patterns between the raised areas. The adhesive is at a level below the peaks of the raised areas and is therefore protected by the raised areas from contact with the undergarment. In an alternative, Lauritzen applies adhesive to the tips of the raised areas of the barrier film and then inverts the raised areas to form depressions. The adhesive is thereby conveniently placed in the depressions. Discrete adhesive placement fails to enable a total adhesive seal. The adhesive must be interconnected to achieve such a seal.
Lauritzen notes that the depressions must be formed such that they are strong enough to protect the adhesive from inadvertent contact during shipping and storage, yet are deformable by pressing the undergarment against the barrier film. Lauritzen states that sufficient rigidity is available from 0.03 inch to 0.06 inch thick film. In one embodiment, conical depressions have diameters at their open ends of 0.1 to 0.5 inches and depths of 0.1 to 0.25 inches. Such large depressions and thick film preclude the ability to wind up the fluid-impervious barrier film separately in a compact a roll.
In still another Lauritzen embodiment, a printing device transfers adhesive to a printing belt, which transfers adhesive to a release belt, and then to the barrier film web. This system is used to first print a pattern of discrete patches of adhesive onto the barrier film web. Then the film web is registered with a vacuum plate. When each patch of adhesive is disposed above a recess in the forming plate, vacuum forms adhesive containing depressions. Because of registration requirements, the invention is believed limited to relatively large depressions.
U.S. Pat. No. 4,959,265 to Wood et al. discloses an adhesively coated substrate having bluntly pointed stems protruding beyond a layer of pressure sensitive adhesive. The back of this substrate may be bonded to a sanitary napkin. The napkin may then be installed onto a foraminous fabric by pressing to force the stems to penetrate the fabric to a depth where the fabric contacts the adhesive. xe2x80x9cBy penetrating fabric, the stems significantly reinforce the adhesion of the fastenerxe2x80x9d. The stems xe2x80x9cshould be resistant to compression and bending, even thought they may be quite supple.xe2x80x9d xe2x80x9cThe backing with its stems preferably is formed from a tough thermoplastic resin by cast molding or extrusion molding.xe2x80x9d The stems protrude 20 microns to 3 mm above the adhesive level, depending on the coarseness of the fabric to be penetrated. The adhesive area is preferably between 3 and 30 times the area occupied by the stems. Smooth rigid surfaces are obviously inoperable with Wood et al.
U.S. Pat. No. 5,344,693 to Sanders discloses a substrate having a plurality of non-interconnecting spacing means extending outwardly from an adhesive coated surface to space the surface from another surface until the surfaces are pressed together. The spacing means of Sanders is non-deformable. Sanders points out that prior art discloses deformable spacing means where force is applied to a surface of interest, but that his spacers are non-deformable so that spacing will be maintained when the substrate is stored in a roil. The spacing members are spaced not more than 80 times the dimension of each spacer. Sanders discusses this invention for use with reclosable bags.
U.S. Pat. No. 5,141,790 to Calhoun et al. discloses one-time repositionable pressure sensitive tapes with an adhesive coated surface having clumps of particles spaced out on the adhesive to keep the adhesive from touching a target surface during positioning and until the sheet is pressed against the target surface. The particles are smaller than the thickness of the adhesive layer so that when pressed, the particles sink below the surface of the adhesive and no longer provide their spacing function. Thus, Calhoun et al. is intended for only one use.
U.S. Pat. No. 4,061,820 to Magid et al. discloses a foam with cells. The foam is compressed to open cell cavities at the foam surface so that pressure sensitive adhesive can be applied to the open cells. When the foam is released, the cells close and hide the adhesive. When the foam is pressed, the cells bring pressure sensitive adhesive to the surface for sticking the foam to a target surface. This is believed to be both an expensive and significantly thick solution to the problem.
The prior art has focused on the use of stand-offs with pressure sensitive adhesives primarily in the area of sanitary napkins. However, different solutions are needed for flexible films intended to be stuck to themselves or to smooth rigid surfaces.
The prior art methods for applying adhesive to surfaces having stand-offs are aimed at surfaces having large sized stand-offs and/or large spacing between stand-offs. What is missing is a method of applying pressure sensitive adhesive to a flexible film with very small stand-offs and which are closely spaced, where the registration of stand-offs and the adhesive pattern is critical.
It is an object of the present invention to provide a composite material having a recessed pressure sensitive adhesive layer and collapsible protrusions which serve as stand-offs to prevent premature sticking to a wide variety of rigid and resilient target surfaces, wherein the collapsible protrusions are small and closely spaced for releasable sealing of the composite material to such surfaces or even to itself.
It is another object of the present invention to provide a composite material having a recessed pressure sensitive adhesive layer and collapsible protrusions which serve as stand-offs, wherein the composite material is relatively thin and therefore inexpensive and capable of being wound onto compact rolls for storage and convenient packaging, yet resistance to total collapse of protrusions is sufficient to prevent premature collapse before the composite material is unwound from a roll for use.
It is still another object of the present invention to provide a composite material having a recessed pressure sensitive adhesive layer and collapsible protrusions which serve as stand-offs, wherein a continuous, leak resistant, adhesive seal may be made by virtue of an interconnected adhesive pattern between protrusions.
It is a further object of the present invention to provide a method for making a composite material having a recessed pressure sensitive adhesive layer and collapsible protrusions, without regard for registration of protrusions and an adhesive pattern, wherein the method is capable of automated continuous motion or indexing motion for a continuous web format.
A flexible film has thermoformed protrusions between a grid of pressure sensitive adhesive. The protrusions prevent adhesion of the film to itself or to another surface until the film is pressed against the target surface. Pressing collapses the protrusions locally to enable the adhesive to contact and stick to the target surface. The adhesive has limited aggressiveness so that the film may be peeled away from the target surface such that the adhesive stays with the film. Even two coated film surfaces may be pressed together to form a seal, yet they may later be peeled apart.
Articles may be wrapped with the film of the present invention, thereby providing a competitive substitute for films that supposedly cling to contact surfaces. Having an adhesive allows a film to stick to plastic containers. Plain plastic films do not stick to such containers. The film could also be used as a tape, which does not stick until pressed.
In one aspect of the present invention, a method for making a flexible film having pressure sensitive adhesive protected from inadvertent adherence coats a forming screen with an interconnected layer of pressure sensitive adhesive and places a piece of flexible film in contact with the interconnected layer of pressure sensitive adhesive on the top surface of the forming screen. The interconnected layer of pressure sensitive adhesive preferentially adheres to the piece of flexible film. The forming screen has a top surface and the top suface has a plurality of recesses therein. The coating step applies the interconnected layer of pressure sensitive adhesive to the top surface without bridging the recesses.
Another step forms the piece of flexible film to create a plurality of protrusions extending into the recesses of the forming screen. The piece of flexible film has an adhesive side and a non-adhesive side. The plurality of protrusions are registered with the interconnected layer of pressure sensitive adhesive on the adhesive side. A final step removes together the piece of flexible film and the layer of interconnected pressure sensitive adhesive from the forming screen.
The forming screen may be wrapped around a rotating drum and the piece of flexible film may be a portion of a continuous web of flexible film, such that the method is a continuous process. Also, the forming screen may be coated with a release material prior to applying the layer of interconnected pressure sensitive adhesive.
The plurality of protrusions may be formed by applying heat to the non-adhesive side of the piece of flexible film and by applying vacuum to the adhesive side of the piece of flexible film. Alternatively, the plurality of protrusions may be formed by applying heated compressed gas to the non-adhesive side of the piece of flexible film while the piece of flexible film rests against the forming screen. In yet another alternative method step, the plurality of protrusions may be formed by mechanically embossing the piece of flexible film into the recesses of the forming screen from the non-adhesive side of the piece of flexible film. Such thermoforming and cold forming methods are well known in the art.
The plurality of protrusions are preferably formed in the piece of flexible film to have a domed shape and each protrusion has a height less than a diameter of the protrusion, so that when pressed, the plurality of protrusions will collapse in a direction substantially perpendicular to a plane of the flexible film.
Each of the plurality of recesses of the forming screen are circular and have a base diameter of about 0.025 inches and depth of about 0.006 inches to 0.012 inches. The plurality of recesses are substantially equally spaced about 0.050 inches apart in an equilateral triangular pattern.
In another aspect of the present invention, a composite material is releasably sealable to a target surface. The composite material comprises a piece of flexible film and pressure sensitive adhesive. The flexible film has an adhesive side and a non-adhesive side. The adhesive side is formed to have a plurality of conical protrusions separated by valleys. The plurality of conical protrusions have outermost ends and a base diameter of 0.015 inches to 0.030 inches, a center-to-center spacing of 0.03 to 0.06 inches, and a protrusion height of 0.004 to 0.012 inches. The valleys are interconnected between the plurality of protrusions. Each of the plurality of protrusions has a domed shape. Each domed shape has a height above the adhesive side of the piece of flexible film less than a diameter of the domed shape. When pressed, the protrusions collapse in a direction substantially perpendicular to the adhesive side without blocking the coating of pressure sensitive adhesive from contact with the target surface. The pressure sensitive adhesive adheres preferentially to the adhesive side of the piece of flexible film and is peelable from the target surface.